with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a weak complex
peak structure with a duration of about 20 sec. The peak count rate
was ~1000 counts/sec (15-350 keV), at ~3 sec after the trigger.
The XRT began observing the field at 04:08:30.6 UT, 141.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 291.33917, -15.42535 which
is equivalent to:

RA(J2000) = 19h 25m 21.40s
Dec(J2000) = -15d 25' 31.3"

with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 119 arcseconds from the BAT onboard position, within the
BAT error circle. This position may be improved as more data are
received; the latest position is available at
http://www.swift.ac.uk/sper. We cannot determine whether the source is
fading at the present time.
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.54
x 10^21 cm^-2 (Willingale et al. 2013).
The initial flux in the 2.5 s image was 4.59e-10 erg cm^-2 s^-1 (0.2-10
keV).
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 147 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag.
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.17.
Burst Advocate for this burst is P. A. Evans (pae9 AT star.le.ac.uk).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB. The source is located at RA, Dec=
19:25:21.40, -15:25:30.2 (J2000, +/-0.5").
We thank the staff of the Observatorio Astron=C3=B3mico Nacional in San Pedro
M=C3=A1rtir. Further observations are planned.

GCN Circular #18121
WeiKang Zheng (UC Berkeley), Xianggao Wang (UC Berkeley, GXU, UNLV)
and Alexei V. Filippenko (UC Berkeley) report on behalf of the
KAIT GRB team:
The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, responded to Swift GRB 150811A (Evans et al.,
GCN 18119) about 10 minutes after the burst. Observations were
performed with an automatic sequence in the clear (roughly R), V,
and I filters, and the exposure time was 20 s per image.
Inside the XRT error circle we detected an un-cataloged source with
coordinates of :

RA = 19:25:21.40 (J2000)
DEC= -15:25:30.96 (J2000)

The object has I band mag of ~16.5 at ~15 minutes after
burst and is decaying. We suggest this is the afterglow of GRB 150811A.
Follow-up observations are encouraged.

GCN Circular #18122
Paul Kuin (MSSL/UCL) and P. A. Evans (U Leicester)
report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 150811A
147 s after the BAT trigger (Evans et al., GCN Circ. 18119). A source
consistent with the XRT position and the optical transient reported
by Butler et al.(GCN Circ. 18120) and Zheng et al. (GCN Circ. 18121)
is detected in the promptly available UVOT exposures.
The preliminary UVOT position is:

RA (J2000) = 19:25:21.4
Dec (J2000) = -15:25:31.1

with an estimated uncertainty of 0.6 arc sec. (radius, 90% confidence).
The transient source is at a distance of 4.6 arcsec of a weak nearby
source.
Preliminary detections for the White and U finding charts using the
UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc.
1358, 373) for the early exposures are:

with an uncertainty of 2.5 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).
This circular was automatically generated, and is an official product of the
Swift-XRT team.

GCN Circular #18125
D. A. Perley (DARK) and S. B. Cenko (GSFC) report:
The Palomar 60-inch automated telescope responded to the trigger for GRB 150811A (Evans et al., GRB 18119) and began observations at 04:10:54 UT, 4.75 minutes after the burst. A sequence of four 60-second i-band images was acquired, followed by a repeating cycle of 60-second r, i, and z-band images, then two epochs of imaging in ri filters later in the night.
The optical afterglow (e.g. Butler et al, GCN 18120; Zheng et al., GCN 18121; Kuin et al., GCN 18122; Japelj et al., GCN 18123) shows a sharp rise in flux during the first few observations, from I ~ 17.5 mag at t_mid = 5.25 minutes to a peak of I ~ 16.2 mag at t = 12.42 minutes (calibrating to USNO B1 I-band standards). Afterwards it levels out and then declines steadily, reaching I ~ 18.3 mag at t = 135.1 min and I ~ 19.0 mag at t = 303.85 min, corresponding to an average decay rate of approximately alpha = 0.8.
Further follow-up is planned. Spectroscopy is highly encouraged.

GCN Circular #18127
D.N. Burrows (PSU), C. Pagani (U. Leicester), A.P. Beardmore (U.
Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia
(ASDC), T.G.R. Roegiers (PSU), L.M. McCauley (PSU) and P.A. Evans
report on behalf of the Swift-XRT team:
We have analysed 8.5 ks of XRT data for GRB 150811A (Evans et al. GCN
Circ. 18119), from 128 s to 27.8 ks after the BAT trigger. The data
comprise 156 s in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Goad et al.
(GCN Circ. 18124).
The light curve can be modelled with an initial power-law decay with an
index of alpha=2.52 (+/-0.19), followed by a break at T+366 s to an
alpha of 1.17 (+0.06, -0.10).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.11 (+0.21, -0.13). The
best-fitting absorption column is 1.60 (+0.65, -0.07) x 10^21 cm^-2,
consistent with the Galactic value of 1.5 x 10^21 cm^-2 (Willingale et
al. 2013). The PC mode spectrum has a photon index of 2.03 (+0.13,
-0.15) and a best-fitting absorption column of 2.1 (+1.0, -0.6) x 10^21
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 3.5 x 10^-11 (4.9 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:

If the light curve continues to decay with a power-law decay index of
1.17, the count rate at T+24 hours will be 3.2 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-13 (1.6 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00651882.
This circular is an official product of the Swift-XRT team.

with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 67%.
The mask-weighted light curve shows a single FRED-like pulse that starts
at ~T+1 s, peaks at ~T+6 s, and ends at ~T+40 s. T90 (15-350 keV)
is 34.00 +- 6.25 sec (estimated error including systematics).
The time-averaged spectrum from T+1.51 to T+40.36 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
2.13 +- 0.27. The fluence in the 15-150 keV band is 4.9 +- 0.8 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+6.59 sec in the 15-150 keV band
is 0.5 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence
level.
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/651882/BA/

GCN Circular #18131
N. P. M. Kuin (UCL-MSSL) and P. A. Evans (U Leicester)
report on behalf of the Swift/UVOT team:
We report further on the Swift/UVOT observations of GRB 150811A which
began 148 s after the BAT trigger (Evans et al., GCN Circ. 18119).
A source consistent with the XRT position (Goad et al. GCN Circ. 18124)
was reported earlier in the optical/ir by Butler et al.(GCN Circ. 18120)
Zheng et al. (GCN Circ. 18121), Kuin et al.(GCN Circ. 12122), Japelj et
al., (GCN Circ. 12123), Perley and Cenko (GCN. Circ. 12125), Moskivitin
(GCN Circ. 12128). Our initial observations in the white filter show a
rise from the start at 147s around magnitude (white) 21 which continues
in the u band event data and starts leveling off around magnitude 17.8
550s after the trigger, consistent with the report by Perley and Cenko.
The data taken during the next orbit show that the decay had already
set in. The uv bands showed marginal count rates in all three bands
initially below the 3-sigma level, suggesting the burst is at low redshift.
Preliminary detections and 3-sigma upper limits using the UVOT photometric
system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early
exposures are:

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB. In comparison with our previous
observations, the source flux has declined approximately as t^(-1).
We thank the staff of the Observatorio Astron=C3=B3mico Nacional in San Pedro
M=C3=A1rtir. Further observations are planned.

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB.
We thank the staff of the Observatorio Astron=C3=B3mico Nacional in San Pedro
M=C3=A1rtir.

These magnitudes are in the AB system and are not corrected for Galactic
extinction in the direction of the GRB. The fading of the source remains
consistent with t^(-1) (see, Butler et al., 18120, 18132, 18140).
We thank the staff of the Observatorio Astron=C3=B3mico Nacional in San P=
edro
M=C3=A1rtir.